1. Field of the Invention
The invention relates to optical systems, and in particular to apparatus for stabilizing the shape and energy of optical pulses.
2. Description of the Prior Art
Various types of optical systems employing optical pulse signals are known in the prior art. High speed optical computing and communication systems require precisely defined pulses. Experimental systems require pulses on the picosecond to nanosecond time scale with specific temporal intensity profiles for triggering, coding, and analysis.
Several approaches to pulse shaping have been proposed in the prior art that generally use either active or passive shaping techniques. Active pulse shaping techniques include electrooptic deflectors and Pockels cells. Passive pulse-shaping techniques include mirror stackers, etalon stackers, intensity dependent filters, flat lens shapers, nonlinear interferometers, birefringent filters and double-grating pulse shaping systems.
In particular, an article entitled "Optical Pulse Shaping With a Grating Pair" by J. Agostinelli, G. Harvey, T. Stone and C. Gabel in Applied Optics, Vol. 18, No. 14, 15 July 1979, pp. 2500-2504 discloses the concept of a passive pulse-shaping system that uses a pair of diffraction gratings along with various filters of amplitude and/or phase type to alter the temporal and/or instantaneous spectral profile of the input pulse. Amplitude filters will attenuate certain spectral components and therefore certain portions of the temporal profile of the input pulse will be attenuated. Phase filters will shift various groups of spectral components in time.
As shown in the article, an unshaped input pulse enters the system by impinging upon a first diffraction grating. The diffracted beam emerges as a diverging fan of rays due to the bandwidth of the input pulse and the dispersive nature of the grating. The diverging beam then impinges upon a second diffraction grating of identical groove spacing as the first grating. The angles of the two gratings are precisely matched, so that after the second diffraction, the rays emerge parallel to the input ray direction. A mirror is set perpendicular to the beam emerging from the second grating in order to reflect light back through the pair of diffraction gratings, with each ray retracing its steps, so that a collimated beam emerges at the output of the system in the opposite direction of the incident beam.
Each spectral component of the input pulse traverses a different distance in passage through this system. However, due to the negatively dispersive nature of the grating pair, high frequency components of the input pulse emerge prior to the lower frequency components.
In the plane of the mirror, called the filter plane, there is both spatial and temporal transposition of the spectral components of the input pulse. Amplitude filters are inserted in the "filter plane" to attenuate certain spectral components and therefore attenuate certain portions of the temporal profile of the output pulse. Phase filters are inserted to shift various groups of spectral components in time.
Further, the article discloses the use of various opaque strips placed at various places in the "filter plane" to alter the shape of the output pulse and the use of a plate having a continuously varying transmission function to produce a linearly ramped output pulse.
Unfortunately, the output pulse from the system shown in the article is linearly frequency modulated, "as predicted by linear systems theory (the Fresnel transform of a band-limited Gaussian is a wider Gaussian with linear FM)". Furthermore, the output pulse is not transform-limited, i.e., the output pulse has more spectral width than is necessary to support the features of the shape of the intensity profile, and it is necessarily longer than the input pulse. Since a transform-limited pulse will propagate a greater distance in an optical fiber than a non-transform-limited pulse before being distorted by dispersion of the group velocity, such a configuration is a substantial drawback in using output pulses from the system disclosed in the article in optical digital communications systems.
Another approach to pulse shaping is described in our U.S. Pat. No. 4,655,547 and assigned to the assignee of the present application. In that optical system, an input optical pulse is chirped, and the chirped pulse is then passed through an optical component that spatially disperses the frequency components of the chirped pulse and partially compensates the chirp.
The spatially dispersed frequency components are then passed through spatial amplitude and/or phase masks that control and/or adjust the amplitude and/or phase of the frequency components. Finally, the masked components are passed through the first or a second optical component that returns the masked, spatially dispersed frequency components substantially to the spatial distribution of the input pulse while substantially completing the compensation of the chirp to form an output pulse.
Although such an optical system permits the creation of very short pulses useful in high speed optical digital communications, there are frequently fluctuations in the width, shape, and energy of pulses formed by such optical systems. Such noise places a limit on the amount of information that may be communicated in such an optical system. Prior to the present invention, there has not been a suitable technique for stabilizing very short optical pulses to the extent necessary for some optical communication system applications.